Thin film photovoltaic (PV) modules (also referred to as “solar panels”) based on cadmium telluride (CdTe) paired with cadmium sulfide (CdS) as the photo-reactive components are gaining wide acceptance and interest in the industry. CdTe is a semiconductor material having characteristics particularly suited for conversion of solar energy to electricity. For example, CdTe has an energy bandgap of about 1.45 eV, which enables it to convert more energy from the solar spectrum as compared to lower bandgap semiconductor materials historically used in solar cell applications (e.g., about 1.1 eV for silicon). Also, CdTe converts radiation energy in lower or diffuse light conditions as compared to the lower bandgap materials and, thus, has a longer effective conversion time over the course of a day or in cloudy conditions as compared to other conventional materials.
The junction of the n-type layer and the p-type layer is generally responsible for the generation of electric potential and electric current when the CdTe PV module is exposed to light energy, such as sunlight. Specifically, the cadmium telluride (CdTe) layer and the cadmium sulfide (CdS) form a p-n heterojunction, where the CdTe layer acts as a p-type layer (i.e., a positive, electron accepting layer) and the CdS layer acts as a n-type layer (i.e., a negative, electron donating layer). Free carrier pairs are created by light energy and then separated by the p-n heterojunction to produce an electrical current.
A resistive buffer layer can sometimes be included between a transparent conductive layer and the cadmium sulfide layer in an attempt to reduce the thickness of the cadmium sulfide layer. This resistive buffer layer can inhibit the formation of interface defects, such as pinholes, that would create localized junctions between the transparent conductive oxide layer and the cadmium telluride layer. The resistive buffer layer can be formed by sputtering deposition, which typically involves ejecting material from a target (i.e., the material source), and depositing the ejected material onto the substrate to form the film. DC sputtering generally involves applying a voltage to a metal target (i.e., the cathode) positioned near the substrate (i.e., the anode) within a sputtering chamber to form a direct-current discharge. The sputtering chamber can have a reactive atmosphere (e.g., an oxygen atmosphere, nitrogen atmosphere, fluorine atmosphere) that forms a plasma field between the metal target and the substrate. When metal atoms are released from the target upon application of the voltage, the metal atoms can react with the plasma and deposit onto the surface of the substrate. For example, when the atmosphere contains oxygen, the metal atoms released from the metal target can form a metallic oxide layer on the substrate.
However, the reactive atmosphere can also react with the metal surface of the target, leading to formation of, for instance, an oxide layer on the target. This oxidized surface of the target can lead to problems during sputtering, especially during commercial-scale manufacturing processing where the same target is used to form a resistive buffer layer on multiple substrates during mass production of CdTe PV devices. Such a build-up of an oxidized surface on the target can lead to inconsistencies throughout the production process. For example, the quality of the resistive buffer layer can vary from device to device during the production process.
Thus, a need exists for methods of manufacturing cadmium telluride photovoltaic devices having substantially uniform resistive buffer layers formed in the device during commercial-scale production.